That's Linux on top of a specific real-time kernel.
Look for Xenomai too.
That's Linux on top of another specific real-time kernel, nowadays submerged in a windy river.
That's Linux with patches in its kernel.
No, those are two approaches (one: make Linux run as a task on top of an RTOS, use directly the RTOS when you need hard RT, but Linux apps when you don't; two: fix Linux kernel to make it more preemptable and less unpredictable, use always Linux applications) to solve the same problem. Each approach with its variations, trade marks and the like.
If you need hard realtime you need to stick with RTL or RTAI. Both have a HAL beneath the linux jernel that provide hard-realtime API. If soft-RT is good enough for you the standard 2.6 kernel preforms pretty well as long as don't hit a shitty driver ported from early 2.4 kernel versions. In 2.6 99% of the kernel code is preemtible (only the first level ISRs are not but those are well written in most cases)
I've used soft RT on 2.6 (and the latest 2.4) kernels without significant problems.